The mechanism by which accommodation occurs in the human eye is currently a matter of controversy, attributed variously to the action of the ciliary muscle through the zonular apparatus, the hydraulic action of the vitreous on tba anterior segment, or some combination thereof. Resolving this controversy is a matter of importance, since the age-dependent loss of accommodative amplitude - presbyopia - can only be understood, and perhaps arrested, through an understanding of the aging of the accommodative mechanism. The long-term goal of this research, then, is to characterize the aging of the acccmmodative process in the human eye. This grant proposal is specifically concerned with accommodation and the development of presbyopia in the rhesus eye. Adult rhesus accommodation and accomodative loss will be modelled analytically, with the forces needed to act on the lens determined from a description of lens deformation during a small accomodation; changes in the magnitude and/or pattern of forces with age will help to define those factors whose aging contributes to accommodative loss. A second type of modelling approach - finite element analysis - will also be employed to expand the scope of the study without the necessity of developing an even more complex analytical formulation. Computer-based ray-tracing studies will be used to characterize image formation in the adult eye, and will be compared to humans. Characterization of the complex elastic properties of the rhesus monkey lens using dynamic shape analogy modelling of its behavior in a harmonic oscillator will also be performed, providing not only the necessary elastic constants for the accommodation models, but also testing directly the hypothesis that presbyopia is due to decreased lens compliance. The results of these studies will be quantitatively compared to the results of an equivalent study on human accommodation and presbyopia, currently underway in the PI's laboratory. The human and rhesus monkey are closely linked phylogenetically, and their visual systems exhibit strong qualitative similarities in both accommodative mechanism and the development of prestyopia, while differing quantitatively in many respects. The proposed study will allow the characterization of factors shared in common between these two species, and provide an insight into the specific role of non-shared qualities (e.g. the number of the zones of discontinuity) in image formation and the focussing process.